It is often necessary to impart tension to a belt in a drive system. For example, where the belt is used for driving a camshaft of an engine, excessive changes in belt tension may occur with wide variations in ambient temperature due to the different coefficients of expansion of the belt and the body of the engine. Vibrations, noise, unmeshing of the teeth, and the like, may result. Accordingly, belt tension must be maintained within a suitable range.
The basic construction and principle of operation of hydraulically actuated belt tensioners are well-known as shown, for example, in U. S. Pat. No. 4,708,696. A piston rod extends from a piston slidably fitted in a cylinder dividing the cylinder into front and rear chambers which are filled with oil. The piston and rod are biased in a protruding direction by a spring.
The pressure within the rear chamber may rise as high as 100 kg/cm.sup.2 when a force to cause the piston rod to retract is imparted by the belt. Since the rear chamber is confined by the cylinder and the piston, oil within the rear chamber is unable to flow out of the cylinder and is only allowed to flow into the front chamber through the small clearance between the cylinder and the piston. Therefore, some oil leakage and infiltration of air may occur.